While the myocardium is the most important muscle structure of the body, it has but recently been studied carefully or well understood clinically or pathologically. A heart was "hypertrophied" or "dilated" or perhaps "fatty." It suffered from "pain," "angina pectoris," from some "serious weakness" or from "coronary disease," and that ended the pathology and the clinical diagnosis. This is the age of heart defects; no one can understand a patient's condition now, whatever ails him, without studying his heart. No one can treat a patient properly now without considering the management of the circulation. No one should administer a drug now without considering what it will do to the patient's heart. Although we are scientifically interested in the administration of specific treatments, antitoxins and vaccines; although we have a better understanding of food values, and order diets with more careful consideration of the exact needs of the individual, and although we are using various physical methods to promote elimination of toxins, poisons and products of metabolism, we have until lately forgotten the physical fact that one thirteenth of the weight of a normal adult is blood. A man who weighs 170 pounds has 13 pounds of blood. This proportion is not true in the obese, and is not true in children. Whether the person is sick in bed, miserable though up and about, or beginning to feel the first sensations of slight incapacity for his life work, his ability properly to circulate this one thirteenth of his weight through the various arterial and venous channels and capillary tracts must, with the increasing tension and speed of our lives, be taken into consideration. The more and more frequently repeated statements that the operation was successfully performed but that the patient died of shock, and that the typhoid fever and the pneumonia were being successfully combated, but that the patient died of heart failure, together with the increase in arteriosclerosis, cardiac disturbances and renal disease, emphatically present the necessity of more carefully studying the circulation. A better understanding and the constant study of the blood pressure shows nothing but the necessity of the age. The unwillingness of the patient to suffer pain, even for a few minutes, without some narcotic, generally a cardiac debilitating drug, means that, if he is a sufferer from chronic or recurrent pain, he has taken a great deal of medicine which has done his heart no good. Repeated high tension of life raises the blood pressure and puts more work on the heart. Therefore the heart is found weary, if not actually degenerated, when any serious accident, medical or surgical, happens to the patient.